Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. These induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, brakes for controllable suspension systems, vibration dampers in controllable power train and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over other types of controllable fluids, such as ER fluids, particularly for automotive applications, because the MR fluids are relatively insensitive to common contaminants found in such environments, and they display large differences in rheological properties in the presence of a modest applied field.
A typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
Because MR fluids contain noncolloidal solid particles which are at least five times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Without some means of stabilizing or suspending the solid, sedimentation and/or flow induced separation of the solid phase from the liquid phase will occur. Such separation will have a drastic and detrimental effect on the ability of the MR fluid to provide optimal and repeatable performance.
The magnetizable particles are kept in suspension by dispersing a thickener or thixotropic agent in the liquid vehicle. There are basically two approaches to the stabilization of MR fluids: the use of polymeric thickeners, such as high molecular weight hydrocarbons, polyureas, etc., or the use of a finely divided solid, such as fumed silica or colloidal clay. Essentially, both approaches aim to prevent separation of the liquid and solid phases by forming a thixotropic network which “traps” or suspends the heavier solid in the lighter liquid.
Fumed silica can be used as a stabilizer in MR fluid compositions, provided attention is given to the selection of fumed silica grades that are compatible with the chemistry of the liquid phase. This selection is complicated by the fact that the liquid phase is often a combination of miscible, but chemically different materials. If adequate shear mixing is achieved in processing, a lightly gelled system can be formulated using fumed silica. Although characterized by a “yield stress” (defined as the applied force/area required to initiate flow) sufficient to prevent settling, it has been shown that such a system will still flow with a moderate to low viscosity. However, one perceived disadvantage in using fumed silica is that this material, even in amounts less than two or three percent/volume, can cause the MR fluid to be abrasive towards polymeric seals as well as metallic wear surfaces in the device. This may be particularly detrimental in vehicle damper applications, where a considerable amount of expense and effort has been devoted to providing wear-resistant coatings, for example, to protect the damper from failure due to excessive wear. Also, there is growing evidence that fumed silica is a key factor contributing to “in-use thickening”, or paste formation, of MR fluids in suspension dampers subjected to accelerated durability testing. Finally, fumed silicas are sensitive to the presence of contaminants, and their ability to form a network can be significantly compromised by certain contaminants.
Surface-treated, colloidal organoclay has also been used as a stabilizer for MR fluids. In contrast to polymeric thickeners, and similar to fumed silica, an MR fluid with an organoclay thickener typically will form a light gel at low volume concentrations, with a yield stress sufficient to prevent or significantly retard settling, but with an ability to flow with low to moderate viscosity. Moreover, the clay is inherently less abrasive than fumed silica, suggesting the possibility to reduce expensive surface treatments used to retard or prevent abrasion. However, organoclay thickeners typically require the use of dispersants such as propylene carbonate and there are some indications that propylene carbonate can result in a decrease in durability for the MR fluid. Accordingly, systems containing organoclays may exhibit poor durability performance due to the presence of dispersants in the organic clay.
MR fluids with 100% water atomized iron and conventional antiwear and antifriction additives may also exhibit unacceptable durability especially in demanding applications. Although not wishing to be bound by theory, it is theorized that the decreased durability in 100% water atomized iron MR fluid systems may be due to particle-particle attritions and/or particle-hardware attrition, resulting in particle fracture and the generation of fines and formation of virgin reactive iron surfaces. These effects can be mitigated to some extent by replacing some of the water atomized iron with soft carbonyl iron.
Therefore, a need exists for a durable MR fluid composition that utilizes a thickener or thixotropic agent that does not present the durability limitations associated with organoclays and/or dispersants such as propylene carbonate. Furthermore, it would be desirable to provide an MR fluid that is durable even though it is based on 100% water atomized iron with little, if any, carbonyl iron.